Renal Transplant Evaluation⁹⁹ᵐTc-MAG3

Quick Answer

Radionuclide renography (Tc-99m-MAG3) assesses a transplant kidney's perfusion and function and helps sort the common causes of early graft dysfunction. The classic patterns: acute tubular necrosis (ATN) — preserved perfusion but poor tubular excretion (retention), typical in the first days; rejection — reduced perfusion and function; obstruction — collecting-system retention relieving with drainage/diuretic; and urine leak/urinoma — progressive extravasated activity outside the collecting system and bladder.

Mechanism & Technique

The transplant kidney sits in the iliac fossa — superficial and anterior, so it is imaged anteriorly with excellent counts. Tc-99m-MAG3 is cleared by tubular secretion with high extraction, giving good images even in early graft impairment. A study has a flow (perfusion) phase — a rapid bolus tracked against the adjacent iliac artery to grade relative perfusion — followed by dynamic function/excretion imaging. A perfusion index or first-minute uptake slope quantifies graft blood flow; the renogram curve and excretion into the bladder grade tubular function and drainage. Serial studies on the same protocol are the strength of the technique.

Patterns of Graft Dysfunction

ATN and rejection overlap early (and CNI toxicity mimics both) — imaging narrows the differential and follows the trend, but biopsy remains definitive for rejection.

Perfusion vs function sorts the differential

Transplant renography (Tc-99m-MAG3) sorts early graft dysfunction by whether perfusion and function move together: ATN — preserved perfusion but poor tubular excretion, early and self-limited (the commonest early cause); acute rejection — both perfusion and function fall (usually later); obstruction — collecting-system retention that relieves with drainage/diuretic; and urine leak — extravasated activity outside the collecting system and bladder, tracking to a urinoma. Serial trends beat any single scan when early features overlap.

A photopenic (absent-perfusion) graft is a vascular emergency — renal-artery or renal-vein thrombosis — needing immediate surgical attention, not observation. Conversely, preserved perfusion with poor excretion is the reassuring ATN pattern that typically recovers. Reporting the reporting checklist: always separate the perfusion finding from the function/excretion finding — that split is what sorts the differential.

CNI (calcineurin-inhibitor) toxicity closely mimics ATN (preserved perfusion, reduced function) and is distinguished by drug levels/biopsy, not scan alone; rejection additionally drops perfusion. Urine leaks can be subtle — look for activity outside the collecting system/bladder tracking to a urinoma, best seen on delayed/post-void images. Imaging narrows and trends; biopsy is definitive for rejection.

High-Yield Pearls

• ATN: good perfusion, bad excretion, early, self-limited — the commonest early cause.
• Rejection: perfusion and function both decline (helps separate from ATN); CNI toxicity mimics ATN.
• Absent perfusion = vascular thrombosis — a surgical emergency.
• Serial studies track recovery/deterioration; biopsy remains definitive for rejection.

Common Pitfalls

• Overlapping ATN/rejection/CNI features early on — trends across serial studies beat a single scan.
• A urine leak can be subtle — look for extravasated activity outside the collecting system/bladder on delayed images.
• Not distinguishing the perfusion from the function abnormality (the core sorting step).

Related Pages

• Tracer: Tc-99m-MAG3; protocol: Diuretic renography, Captopril renography.

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